proportional problem

Question

A department store sold 3 pairs of jeans for $80.  Last week Mrs. Denton bought 7 pairs of jean at the store. Which proportion can be used to find C, the cost of the 7 pairs of jeans?  Using this information, what would the equation look like to find C?

Answer 1

Set up 2 fractions, cost/#of jeans and set them equal to each other:

 

$80/3 jeans = C/7 jeans  *The important thing is to have the units of both fractions match

 

Then you cross multiply.  3C=80 x 7  and solve for C